Cyclic AMP and cyclic GMP phosphodiesterase inhibition by an antiplatelet agent, 6-[(3-methylene-2-oxo-5-phenyl-5-tetrahydrofuranyl)methoxy)quinol inone (CCT-62)

Progress in the development of antiplatelet agents: Focus on the targeted molecular pathway from bench to clinic

Antiplatelet drugs serve as a first-line antithrombotic therapy for the management of acute ischemic events and the prevention of secondary complications in vascular diseases. Numerous antiplatelet therapies have been developed; however, currently available agents are still associated with inadequate efficacy, risk of bleeding, and variability in individual response. Understanding the mechanisms of platelet involvement in thrombosis and the clinical development process of antiplatelet agents is critical for the discovery of novel agents. The functions of platelets in thrombosis are regulated by two major mechanisms: the interaction between surface receptors and their ligands, and the downstream intracellular signaling pathways. Recently, most of the progress made in antiplatelet drug development has been achieved with P2Y receptor antagonists. Additionally, the usage of GP IIb/IIIa receptor antagonists has decreased, because it is associated with a higher risk of bleeding and thrombocytopenia. Agents targeting other platelet surface receptors such as PARs, TP receptor, EP3 receptor, GPIb-IX-V receptor, P-selectin, as well as intracellular signaling factors, such as PI3Kβ, have been evaluated in an attempt to develop the next generation of antiplatelet drugs, reduce or eliminate interpatient variability of drug efficacy and significantly lower the risk of drug-induced bleeding. The aim of this review is to describe the pathways of platelet activation in thrombosis, and summarize the development process of antiplatelet agents, as well as the preclinical and clinical evaluations performed on these agents.

Novel thienylacylhydrazone derivatives inhibit platelet aggregation through cyclic nucleotides modulation and thromboxane A2 synthesis inhibition

The aim of this study has been to investigate the antiplatelet activity of a new series of thienylacylhydrazone derivatives analogous to the lead compound LASSBio-294 ((2-thienylidene) 3,4-methylenedioxybenzoylhydrazine). The antiplatelet effect was investigated in rabbit and human platelet rich plasma stimulated by arachidonic acid, collagen, ADP and in washed platelet stimulated by thrombin. The effects on the production of cyclic nucleotides and thromboxane A(2) (TXA(2)) in human platelets were also investigated. Compounds LASSBio-785 (N-Methyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-786 (N-Benzyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-787 ((5-Methyl-2-thienylidene) 3,4-methylenedioxybenzoylhydrazine), LASSBio-788 (N-Allyl (2-thienylidene) 3,4-methylenedioxybenzoylhydrazine) and LASSBio-789 ((5-Bromo-2-thienylidene) 3,4-methylenedioxybezoylhydrazine) inhibited platelet aggregation induced by arachidonic acid, collagen and ADP. LASSBio-785, LASSBio-788 and LASSBio-789 presented the higher potency in platelet aggregation induced by arachidonic acid (IC(50) values of 0.3, 0.2 and 3.1 microM, respectively) and collagen (IC(50) values of 0.9, 1.5 and 3.4 microM, respectively), with a 20 to 70-fold increase in potency compared to LASSBio-294. They inhibited the ATP release reaction by 95%, the whole blood aggregation by 35-45% and the TXB(2) production was totally abolished. In addition, they presented a significant effect on bleeding time. Qualitative studies in thrombin-induced washed platelet aggregation in the presence of sodium nitroprusside (SNP) suggested a phosphodiesterase-2 (PDE2) like effect for LASSBio-785, LASSBio-788 and LASSBio-789. They were able to increase the cGMP levels in non-stimulated platelets, in SNP-stimulated platelets and in the presence of 1-H- [1, 2, 4] oxadiazolo [4, 3- a] quinoxalin- 1- one (ODQ). The antiplatelet aggregation activity exerted by thienylacylhydrazone derivatives seems to be related to cyclic nucleotides regulation and TXA(2) synthesis inhibition. The structural modification of compound LASSBio-294 led to the optimization of its pharmacological properties and to the discovery of new potent antiplatelet prototypes with an antithrombotic potential.

Inhibitory mechanisms of dihydroginsenoside Rg3 in platelet aggregation: Critical roles of ERK2 and cAMP

Ginsenoside Rg3, a single ginseng saponin, is known to be a major anti-platelet component of protopanaxadiol that is isolated from Korean red ginseng. In this study, we investigated whether dihydroginsenoside Rg3, a stable chemical derivative of ginsenoside Rg3, also demonstrated anti-platelet activity. Dihydroginsenoside Rg3 inhibited thrombin-induced platelet aggregation in a concentration-dependent manner with an IC50 (concentration producing 50% inhibition) of 18.8 ± 0.4 μM. Ginsenoside Rg3 inhibited platelet aggregation which was induced by thrombin (0.1 U mL−1) with an IC50 of 40.2 ± 0.9 μM. We next determined whether dihydroginsenoside Rg3 affected different types of ligand-induced platelet aggregation. We found that dihydroginsenoside Rg3 inhibited collagen-induced platelet aggregation with an IC50 of 20.0 ± 0.9 μM. To elucidate the inhibitory mechanism of dihydroginsenoside Rg3 on aggregation, we analysed its downstream signalling pathway. It was interesting to note that dihydroginsenoside Rg3 elevated cyclic AMP production in resting platelets, but did not affect cyclic GMP production. In addition, we found that dihydroginsenoside Rg3 potently suppressed phosphorylation of extracellular signal-regulated kinase 2 (ERK2), which was stimulated by collagen (2.5 μg mL−1), but not of p38 mitogen-activated protein kinase. Taken together, our results indicate that dihydroginsenoside Rg3 potently inhibited platelet aggregation via the modulation of downstream signalling components such as cAMP and ERK2.

Flavonoids and ent-labdane diterpenoids from Andrographis paniculata and their antiplatelet aggregatory and vasorelaxing effects

Two new flavones, designated as andropaniculosin A (1) and andropaniculoside A (2), and 30 known compounds were isolated as a result of detailed chemical examination on the whole plants of Andrographis paniculata. Their structures have been elucidated mainly by 1D and 2D NMR, and MS spectroscopic methods. Among them, four flavonoids showed potent inhibition of collagen, arachidonic acid, thrombin, and platelet activation factor induced platelet aggregation. Furthermore, a diterpenoid demonstrated moderate vasorelaxing effect in isolated rat thoracic aorta.

Modulation of human monocyte-derived dendritic cells maturation by a soluble guanylate cyclase activator, YC-1, in a cyclic nucleotide independent manner

This study evaluated how YC-1, a guanylate cyclase activator, affects the maturation of human monocyte-derived dendritic cells. Maturation markers and intracellular signaling pathways were evaluated. YC-1 inhibited the lipopolysaccharide up-regulation of mature markers, including CD40, CD80 or CD86 in a concentration-dependent manner with IC(50) values of 4.6+/-0.4, 4.9+/-0.6 or 4.5+/-0.5 microM, respectively. YC-1, at a higher concentration, inhibited lipopolysaccharide-induced HLADR expression. These effects of YC-1 were not reversed by ODQ (10 microM), which is a soluble guanylate cyclase inhibitor, nor by KT5823 (1 microM), which is a PKG inhibitor. Additionally, YC-1 did not increase levels of cyclic nucleotides in dendritic cells, supporting the claim that YC-1 affects dendritic cells maturation in a cGMP-independent manner. YC-1, in a cGMP-independent manner, inhibited lipopolysaccharide-induced Akt activation, IkappaBalpha degradation and NF-kappaB translocation, all of which are associated with co-stimulatory molecules expression. YC-1 inhibited the capacity of dendritic cell to activate allogenic T cells with an IC(50) value of 1.2+/-0.3 microM. YC-1-treated dendritic cells have mature phenotypes that exhibit up-regulated CCR7, enhanced IL-10 release and low phagocytosis activity in the presence of lipopolysaccharide. In conclusion, YC-1 inhibited the lipopolysaccharide-induced co-stimulatory molecular expression of dendritic cells by inhibiting Akt activation, IkappaBalpha degradation and NF-kappaB translocation. These inhibitory effects on co-stimulatory molecules suppressed the capacity of dendritic cells to activate allogenic T cells. Additionally, YC-1 treated dendritic cells exhibit the up-regulation of CCR7, enhanced IL-10 release and the down-regulation of phagocytosis in the presence of lipopolysaccharide. Accordingly, YC-1 might be a useful tool for evaluation of dendritic cells on autoimmune or allergic disease.

Synthesis, antiproliferative, and vasorelaxing evaluations of coumarin α-methylene-γ-butyrolactones

Certain coumarin alpha-methylene-gamma-butyrolactones were synthesized and evaluated for antiproliferative and vasorelaxing activities. These compounds were synthesized via alkylation of hydroxycoumarins 2a-f followed by oxidation and the Reformatsky-type condensation. The results of this study are as follows (1) for the vasorelaxing activity, coumarin-7-yl alpha-methylene-gamma-butyrolactone 6d, with an IC50 value of 9.4 microM against pig coronary arterial contraction induced by KCl, is a more active vasorelaxant than its coumarin-4-yl counterpart 6a and its gamma-methyl congener 1. A methyl group substituted at C-4 of the coumarin-7-yl moiety reduced the vasorelaxing effect (6d vs 6e) while the 3,4,8-trimethyl derivative 6f was inactive. (2) For the antiproliferative activity, coumarin-4-yl alpha-methylene-gamma-butyrolactone 6a, which exhibited the most potent antiproliferative activity on the growth of MCF7, NCI-H460, and SF-268 with IC50 values of 6.97, 14.68, and 8.36 microM, respectively, is more cytotoxic than its coumarin-7-yl counterpart 6d and the 6,7-dimethyl derivative 6b. For the coumarin-7-yl derivatives, 6d is more active than its gamma-methyl congener 1, indicating that substitution at the gamma-position decreased cytotoxicity.

Antiplatelet effects of KW-7, a new inhibitor of cyclic nucleotide phosphodiesterases

The antiplatelet effect of a new synthetic compound, 8,9-dimethoxyl-1-(4-methoxy-phenyl)-5,6-dihydro-pyrrolo[2,1-a]isoquinoline-2,3-dione (KW-7), was determined in rabbit platelets. KW-7 concentration-dependently prevented platelet aggregation caused by arachidonic acid, collagen, platelet-activating factor, and thrombin. KW-7 induced a substantial increase in cyclic AMP levels and a smaller increase in cyclic GMP levels in platelets. In platelet homogenates, KW-7 inhibited both cyclic AMP- and cyclic GMP-phosphodiesterase activities. The antiplatelet effect of KW-7 was reversed by SQ22536 (an inhibitor of adenylate cyclase) and H89 (an inhibitor of protein kinase A) but not by ODQ (an inhibitor of soluble guanylate cyclase). These data suggest that the antiplatelet effect of KW-7 is cyclic AMP-dependent, and is through inhibition of platelet phosphodiesterases. In addition, KW-7 inhibited arachidonic acid-stimulated thromboxane production; this effect was associated with an increase in prostaglandin D(2) levels indicating KW-7 is also an inhibitor of thromboxane synthase. The dual inhibition of KW-7 on phosphodiesterase and thromboxane synthase might provide an attractive target in developing antiplatelet drugs.

Initial accumulation of platelets during arterial thrombus formation in vivo is inhibited by elevation of basal cAMP levels

Platelet accumulation at sites of vascular injury is the primary event in arterial thrombosis. Initial platelet accrual into thrombi is mediated by interactions of platelet adhesion receptors with ligands on the injured endothelium or in the sub-endothelial matrix. The role of intracellular signals in initial platelet accumulation at sites of endothelial injury, however, is the subject of debate. We have used a newly discovered inhibitor of phosphodiesterase 3A (PDE3A) and the well-characterized PDE3A inhibitor, cilostazol, to modulate 3',5'-cyclic adenosine monophosphate (cAMP) levels in an in vivo model that enables the kinetic analysis of platelet accumulation. These studies demonstrate that elevation of basal cAMP levels results in an overall decline in platelet accumulation at the site of vascular injury. In particular, the initial rate of accumulation of platelets is inhibited by elevation of cAMP. Analysis of the kinetics of individual platelets at injury sites using intravital microscopy demonstrates that cAMP directs the rate at which platelets attach to and detach from thrombi. These studies demonstrate that cAMP in circulating platelets controls attachment to and detachment from sites of arteriolar injury. Thus, the status of the intracellular signaling machinery prior to engagement of platelet receptors influences the rate of platelet accumulation during thrombus formation.

Novel epinephrine and cyclic AMP-mediated activation of BCAM/Lu-dependent sickle (SS) RBC adhesion

The vasoocclusive crisis is the major clinical feature of sickle cell anemia, which is believed to be initiated or sustained by sickle (SS) red blood cell (RBC) adhesion to the vascular wall. SS RBCs, but not unaffected (AA) RBCs, adhere avidly to multiple components of the vascular wall, including laminin. Here we report a novel role for epinephrine and cyclic adenosine monophosphate (cAMP) in the regulation of human SS RBC adhesiveness via the laminin receptor, basal cell adhesion molecule/Lutheran (BCAM/Lu). Our data demonstrate that peripheral SS RBCs contain greater than 4-fold more cAMP than AA RBCs under basal conditions. Forskolin or the stress mediator epinephrine further elevates cAMP in SS RBCs and increases adhesion of SS RBCs to laminin in a protein kinase A (PKA)-dependent manner, with the low-density population being the most responsive. Epinephrine-stimulated adhesion to laminin, mediated primarily via the beta 2-adrenergic receptor, occurred in SS RBC samples from 46% of patients and was blocked by recombinant, soluble BCAM/Lu, implicating this receptor as a target of cAMP signaling. Thus, these studies demonstrate a novel, rapid regulation of SS RBC adhesion by a cAMP-dependent pathway and suggest that components of this pathway, particularly PKA, the beta 2-adrenergic receptor, and BCAM/Lu, should be further explored as potential therapeutic targets to inhibit SS RBC adhesion.

KF31327, a new potent and selective inhibitor of cyclic nucleotide phosphodiesterase 5

The effects of KF31327 (3-ethyl-8-[2-(4-hydroxymethylpiperidino)benzylamino]-2,3-dihydro-1H-imidazo[4,5-g]quinazoline-2-thione dihydrochloride) on phosphodiesterase 5 (cyclic GMP-specific phosphodiesterase) activity and platelet aggregation were investigated and compared with those of sildenafil, a well-known phosphodiesterase 5 inhibitor. KF31327 inhibited phosphodiesterase 5 from canine trachea (K(i)=0.16 nM) more potently than sildenafil (K(i)=7.2 nM). The kinetic analysis revealed that KF31327 was a non-competitive inhibitor. In the presence of nitroglycerin (nitric oxide generator), both compounds inhibited the collagen-induced aggregation of rabbit platelets at less than 0.1 microM, augmenting intracellular cyclic GMP level without affecting cyclic AMP. In contrast, in the absence of nitroglycerin, a higher concentration (10 microM) of KF31327 was required to inhibit platelet aggregation and increased both cyclic nucleotide levels. However, 10 microM sildenafil did not affect aggregation despite elevation of cyclic GMP comparable to that in the presence of nitroglycerin. These results indicate that in the presence of nitroglycerin, the inhibition of platelet aggregation by KF31327 is due to the elevation of cyclic GMP, whereas the mechanism underlying the inhibition without nitroglycerin might be related to a rise in intracellular cyclic AMP.

A sensitive assay of human blood platelet cyclic nucleotide phosphodiesterase activity by HPLC using fluorescence derivatization and its application to assessment of cyclic nucleotide phosphodiesterase inhibitors

A selective and sensitive HPLC measurement of 3',5'-cyclic nucleotide phosphodiesterase (PDE) activity in human platelets using (3,4-dimethoxyphenyl)glyoxal (DMPG) as a fluorogenic reagent for guanine and its nucleosides and nucleotides is described. cGMP, a substrate for PDE, and GMP, which was produced by the enzyme reaction, are selectively converted by the reaction with DMPG to the fluorescent derivatives. The derivatives were separated by reversed-phase HPLC. Human platelet PDE activity was measured and the inhibitory effects of several compounds were investigated.